1) Field of the Invention
The present invention relates to friction welding and, more specifically, to a method for overlapping friction stir weld joints to form multi-pass friction stir weld joints.
2) Description of Related Art
Friction stir welding is a process in which a rotating tool, such as a pin or probe, is urged into and/or through a workpiece, e.g., to join multiple members of the workpiece in a solid state or to repair cracks in a workpiece. Typically, the pin extends from a shoulder, which can be flat, concave, or otherwise contoured, and the shoulder is urged against the workpiece so that the pin is urged therein. The pin is then urged through the workpiece to form a continuous weld joint. For example, during one conventional friction stir welding process, a friction stir welding machine urges the probe of the rotating tool into an interface defined by two abutting workpiece members, and moves the tool along the interface. The motion of the rotating tool generates frictional heating, thereby forming a region of plasticized material in the workpiece. The tool can be tilted approximately 2.5° relative to the workpiece such that the trailing edge of the shoulder is thrust into and consolidates the plasticized material. Upon solidification of the plasticized material, the members of the workpiece are joined along the weld joint. Friction stir welding is further described in U.S. Pat. No. 5,460,317 to Thomas et al., the contents of which are incorporated herein by reference.
Friction stir weld joints are typically not uniform throughout due, at least in part, to the asymmetry of the conventional friction stir welding process. In particular, a conventional friction stir weld joint is characterized by transversely opposite advancing and retreating sides. As shown in FIG. 1, a friction stir weld pin 10 is rotated and advanced longitudinally through a workpiece 12, i.e., two overlapping structural members 20, 22, to form a friction stir weld lap joint 14. The pin 10 is rotated in a direction 26 and advanced longitudinally in a direction 28. The relative speed of the outer surface of the pin 10 with respect to the workpiece 12 is a function of the rotational speed of the pin 10 and the speed of the longitudinal advancement of the pin 10 through the workpiece 12. In particular, the speed of the pin 10 relative to the workpiece 12 at a first side 16, referred to as the advancing side, is generally equal to the sum of the speed of the longitudinal movement of the pin 10 and the product of the rotational, or angular, speed of the pin 10 and the radius of the pin 10. Similarly, the speed of the pin 10 relative to the workpiece 12 at the second side 18, referred to as the retreating side, is generally equal to the product of the rotational speed of the pin 10 and the radius of the pin 10 less the speed of the longitudinal movement of the pin 10. Thus, the relative speed is greater at the advancing, first side 16 of the joint 14, and slower at the retreating, second side of the joint 18.
The material of the workpiece 12 at the advancing side 16 of the joint 14 is typically displaced more and mixed more thoroughly than the material at the retreating side 18 of the joint 14. FIG. 2 illustrates a lap weld joint 14 formed between two anodized aluminum structural members 20, 22, with the friction stir welding pin 10 rotating at 625 revolutions per minute and advanced longitudinally at a speed of 7.5 inches per minute. FIGS. 3 and 4 illustrate portions at the advancing and retreating sides 16, 18, respectively, of the joint 14. The material at the advancing side 16 exhibits better dispersion as compared to the material at the retreating side 18. In particular, the interface 24 between the anodized surfaces of the structural members 20, 22 is still evident, albeit deformed, at the retreating side 18. In contrast, the material at the advancing side 16 is mixed so that the interface 24 no longer exists. The incomplete mixing of the material at the retreating side 18 of the joint 14 can reduce the strength of the joint 14, especially where the interface 24 of the structural members 20, 22 extends from the joint 14, as is the case in a friction stir welded lap joint.
Thus, conventional friction stir welding methods have been shown to form weld joints in which the material of the workpiece is plasticized and mixed, and the resulting granular structure in the weld joint is refined. However, a need continues to exist for an improved friction stir weld joint in which the material has been sufficiently mixed and refined throughout and for an improved method for forming such joints. Preferably, the method should be capable of forming a lap joint, in which the joint extends substantially perpendicular through the interface.